1. Field of the Invention
The present invention relates to a noise elimination method and a noise elimination apparatus for detecting and eliminating a noise signal mixed into an image signal captured at an image pickup sensor of a digital camera or the like.
2. Description of the Background Art
FIG. 10 is a block diagram showing a schematic configuration of a common digital still camera. As shown in the figure, in a digital camera 100, an image signal captured by an image pickup sensor 105 such as CCD sensor or CMOS sensor is converted into a digital signal by A/D conversion, and then subjected to a variety of image processings such as pixel interpolation, color space conversion, edge enhancement at an image processing part 106. The image data after subjected to such image processings is then displayed on a LCD (Liquid Crystal Display) 109 or the like, compression-encoded in the JPEG (Joint Photographic Experts Group) format, for example, and stored in a memory card 110 such as a nonvolatile memory, or outputted to an external device such as a personal computer via an interface 111. In FIG. 10, a reference numeral 101 denotes an optical lens, a reference numeral 102 denotes a color correction filter, a reference numeral 103 denotes an optical LPF (Low-Pass Filter), a reference numeral 104 denotes a color filter array, a reference numeral 107 denotes a driving part for driving and controlling the image pickup sensor 105 and the like.
However, this configuration raises the problem that noise signals get mixed into the image signal captured at the image pickup sensor 105 during the A/D conversion, pixel interpolation and the like processes, to deteriorate the image quality. If such an image signal including noise signals is subjected to an edge enhancement process, the noise signals are also enhanced, to cause the problem of further deteriorating the image quality. Such kinds of noise signals may result from the hardware configuration of the A/D converter, or result from interpolating a current pixel using peripheral pixel values in the pixel interpolation process. Especially in the frequency components in the vicinity of the Nyquist frequency, noise signals 122, 122, . . . distributed in a dotted pattern as shown in FIG. 11 and noise signals 123, 123, . . . distributed in a checkered pattern as shown in FIG. 12 are likely to appear. FIG. 11 and FIG. 12 are schematic views showing an image signal 120 made up of a plurality of pixels 121, 121, . . . arranged in matrix. In the example shown by FIG. 11, the noise signals 122, . . . having signal levels higher than the normal signal level in the other pixels 121, . . . appear in a dotted pattern, and in the example shown by FIG. 12, the noise signals 123, . . . having signal levels higher than the normal signal level in the other pixels 121, . . . appear in a checkered pattern.
Since it is difficult to eliminate such noise signals individually, a LPF (Low-Pass Filter) which allows passage of low frequency components and reduction of high frequency components is caused to act on the entire image signal so as to blur the noise signals. However, according to this measure, also the edge portions are blurred as well, leading the problem that the resolution power is deteriorated.